Astronaut Scott Kelly spent a year living on the International Space Station testing human limits for space travel while his identical twin, retired astronaut Mark Kelly, was monitored here on Earth. In the Kelly brothers, NASA has a near-perfect, two person sample group for biomedical research. This is the story of what it takes to spend a year in space and then, using what NASA has learned from the brothers, what it means for humanity as we journey to Mars and beyond.